X-ray computed tomography (CT) imaging, in its simplest expression, is an X-ray beam traversing an object and a detector relating the overall attenuation per ray. The attenuation is derived from a comparison of the same ray with and without the presence of the object. From this conceptual definition, several steps are required to properly construct an image. For instance, the finite size of the X-ray generator, the nature and shape of the filter blocking the very low energy X-ray from the generator, the details of the geometry and characteristics of the detector, and the capacity of the acquisition system are all elements that affect how the actual reconstruction is performed.
For a combined 3rd-generation and 4th-generation CT scanner, the system matrix requires knowledge of the offset of the 4th-generation sparse photon-counting detectors relative to the 3rd-generation CT system coordinates. If the offset between the 3rd-generation isocenter and the 4th-generation detectors is not know precisely, the system matrix will be incorrect, which will cause artifacts in the reconstructed image. Moreover, the offset could change over time due to temperature-induced expansion of materials and gantry rotation.